Tribological Behavior of Babbitt Alloy Rubbing Against Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and Steel Under Dry Friction Condition

The tribological behavior of Babbitt alloy rubbing with Si₃N₄ ball and steel ball with various sliding speeds at dry friction condition was investigated. It was found that B88 alloy rubbing with Si₃N₄ ball and steel ball possesses a low sliding wear resistance at dry friction. The wear rate is above 10^?4 mm³/Nm, and the friction coefficient is from 0.2 to 0.4. At low sliding speed of 0.05-0.1 m/s, the mainly wear mechanisms are microgroove and fatigue wear, while at high sliding speed of 0.5 m/s, the wear mechanisms depend on plastic deformation and delamination. The high wear rate indicates that it is needed to prevent Babbitt alloy from working at dry friction conditions, while the low friction coefficient suggests that it is not easy to the occurrence of cold weld.     

Tribological Properties of Ni<Subscript>3</Subscript>Al Matrix Composite Sliding Against Si<Subscript>3</Subscript>N<Subscript>4</Subscript>, SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> at Elevated Temperatures

The Ni₃Al matrix self-lubricating composite was fabricated by powder metallurgy technique. The tribological behavior of the composite sliding against commercial Si₃N₄, SiC and Al₂O₃ ceramic balls was investigated from 20 to 1000 °C. It was found that the composite demonstrated excellent lubricating properties with different friction pairs at a wide temperature range, which can be attributed to the synergetic effect of Ag, fluorides, and molybdates formed by oxidations. The Ni₃Al matrix self-lubricating composite/Si₃N₄ couple possessed the stable friction coefficient and wear rate.     

Diffraction Patterns and Crystal Structure of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and Ge<Subscript>3</Subscript>N<Subscript>4</Subscript>

A nitride, believed to be SiN,, has been separated from three nitrided silicon steels. Germanium nitride, GeN,, has been prepared from pure germanium. Comparison of the diffraction patterns indicates that the two nitrides are isomorphous; an orthorhombic structure is suggested in place of the rhombohedral structure previously reported for Ge₃N₄.     

海水润滑条件下316L不锈钢与仿生非光滑表面CF/PEEK的摩擦学性能

为探讨高压海水轴向柱塞泵滑靴副的减阻抗磨机理,采用数控机床在CF/PEEK试样表面加工出不同形状仿生非光滑单元体,与316L不锈钢形成配副,对其在不同法向载荷下的摩擦因数、试样温度、磨损率等进行测试,并用激光共焦显微镜及扫描电子显微镜对磨损表面进行分析。结果表明:在海水润滑条件下,光滑表面配副以磨料磨损和粘着磨损为主,摩擦因数随时间稳定在0.05~0.09,试样温升大,磨损率大;非光滑表面配副可有效存储海水和磨屑,产生动压润滑效应、降低磨料磨损,摩擦以犁沟效应为主,摩擦因数稳定在0.02~0.06,试样温升小,磨损率小;随法向载荷的增加,由粘着效应所致CF/PEEK转移加快,摩擦因数和试样温度升高,磨损率降低且下降趋势逐渐减缓。     

The creep behavior of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/SiC nanocomposites

In an investigation of the creep properties of silicon nitride/silicon carbide nanocomposites, the micro-nano type composites with nano-SiC at intragranular and inter-granular regions show behavior not much different from that of silicon-nitride monolith. The improvement of creep resistance is modest, up to about one order of magnitude decrease in steady-state creep rate. The creep rate parameters such as activation energy and stress exponent measured for this type of nanocomposite are within the range of those of silicon nitride. This evidence suggests that a special strengthening mechanism may not be necessary for this type of material. Nano-nanocomposites show remarkably lower creep rates, possibly pointing to a new creep mechanism such as solid-state diffusion. For more information, contact A.K. Mukherjee, University of California-Davis, Department of Chemical Engineering and Materials Science, Davis, CA 95616; e-mail akmukherjee@ucdavis.edu.     

Effects of residual stress on fracture strength of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/stainless steel joints with a Cu-interlayer

The variation in fracture strength of a brazed Si₃N₄/Cu/steel joint was compared with the change in residual stress as a function of the Cu-interlayer thickness that was used. The higher residual stress and the lower measured fracture strength for the joint, using a 0.1 mm thick Cu-interlayer, were ascribed to the entire dissolution of the Cu-interlayer into the brazing alloy. The finite element analysis of residual stress, which considered the microstructure at the interface region, could explain the fracture behavior for the brazed joints, which is dependent on the thickness of the Cu-interlayer.     

Dry Sliding Tribological Studies of AA6061-B<Subscript>4</Subscript>C-Gr Hybrid Composites

The dry sliding behavior of stir-cast AA6061-10 wt.% B₄C composites containing 2.5, 5 and 7.5 wt.% graphite particles was studied as a function of applied load, sliding speed and sliding distance on a pin-on-disk tribotester. The wear rate and friction coefficient increased with increase in applied load and sliding distance. The increase in graphite addition reduced the increase in wear rate and friction coefficient in the sliding speed range 2-2.5 m/s. Scanning electron microscopy of the worn pin revealed a graphite tribolayer, and transmission electron microscopy revealed overlapping deformation bands under 30 N applied load. Upon increasing the applied load to 40 N, welded region with fine crystalline structure was formed due to dynamic recrystallization of AA6061 alloy matrix.     

Thermal Shock Resistance of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/h<Emphasis Type="Italic">-</Emphasis>BN Composites Prepared via Catalytic Reaction-Bonding Route

Si₃N₄/h-BN ceramic matrix composites were prepared via a catalytic reaction-bonding route by using ZrO₂ as nitridation catalyst, and the water quenching (fast cooling) and molten aluminum quenching tests (fast heating) were carried out to evaluate the thermal shock resistance of the composites. The results showed that the thermal shock resistance was improved obviously with the increase in h-BN content, and the critical thermal shock temperature difference (ΔT _c) reaches as high as 780 °C when the h-BN content was 30 wt.%. The improvement of thermal shock resistance of the composites was mainly due to the crack tending to quasi static propagating at weak bonding interface between Si₃N₄ and h-BN with the increase in h-BN content. For the molten aluminum quenching test, the residual strength showed no obvious decrease compared with water quenching test, which could be caused by the mild stress condition on the surface. In addition, a calculated parameter, volumetric crack density (N _f), was presented to quantitative evaluating the thermal shock resistance of the composites in contrast to the conventional R parameter.     

Effect of Nano-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-8YSZ Ceramic Coatings

In this paper, the effect of nano-Si₃N₄ additives and plasma treatment on the wear behavior of Al₂O₃-8YSZ ceramic coatings was studied. Nano-Al₂O₃, nano-8YSZ (8 wt.% Y₂O₃-stabilized ZrO₂) and nano-Si₃N₄ powders were used as raw materials to fabricate four types of sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The surface morphologies of the ceramic coatings were observed. The mechanical properties of the ceramic coatings were measured. The dry sliding wear behavior of the Al₂O₃-8YSZ coatings with and without Si₃N₄ additives was studied. Nano-Si₃N₄ additives and plasma treatment can improve the morphologies of the coatings by prohibiting the initiation of micro-cracks and reducing the unmelted particles. The hardness and bonding strength of AZSP (Al₂O₃-18 wt.% 8YSZ-10 wt.% Si₃N₄-plasma treatment) coating increased by 79.2 and 44% compared to those of AZ (Al₂O₃-20 wt.% 8YSZ) coating. The porosity of AZSP coating decreased by 85.4% compared to that of AZ coating. The wear test results showed that the addition of nano-Si₃N₄ and plasma treatment could improve the wear resistance of Al₂O₃-8YSZ coatings.     

Simulation and Analysis of Residual Stress in the Graded Interlayer of MoSi<Subscript>2</Subscript> Composite/316L Stainless Steel Joint

We reported the simulation and the analysis of the residual stress within the graded interlayered joints of MoSi₂ to 316L stainless steel fabricated by spark plasma-sintering (SPS) technique. The residual stress in the joints was analyzed by the finite element ANSYS code. The results show that the maximum radial and axial residual tension stresses first decrease and then increase with the increase of the compositional distribution exponent (P), and consistently decrease with the increase of the number (n) and the thickness (d) of graded layer, respectively. The optimal values of compositional distribution exponent, the thickness, and the number of graded interlayered joints of the MoSi₂ composite/316L are P = 0.8, d = 1.0 mm, and n = 9, respectively. The residual tension stresses in the samples treated by nine graded interlayers are reduced to 24% and 25% of those in the joints without the interlayer, respectively.     

HAP Coatings for Biomedical Applications: Biocompatibility and Surface Protection Against Corrosion of Ti,Ti6Al4V and AISI 316L SS

Protection of Metals and Physical Chemistry of Surfaces - The implants used as cardio stents, orthopedic and dental implant may be subjected to biological corrosion. Uncoated implants can be...     

Rapid consolidation of nanocrystalline NbSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> composites by pulsed current activated combustion synthesis

A dense nanostructured 4NbSi₂-Si₃N₄ composite was synthesized by a Pulsed Current Activated Combustion Synthesis (PCACS) method within 3 minutes in one step from mechanically activated powders of NbN and Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of a pulsed current and mechanical pressure. A highly dense 4NbSi₂-Si₃N₄ composite with a relative density of up to 98 % was produced under simultaneous application of a 60 MPa pressure and the pulsed current. The average hardness and fracture toughness values obtained were 700 kg/mm² and 3.5 MPa·m^1/2, respectively     

Influence of processing parameters on the phase composition of ZrN-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> synthesized from zircon

The optimum parameters were determined for synthesizing ZrN-Si₃N₄ composite powder from zircon by carbothermal reduction-nitridation (CTRN) process. The samples were prepared by mixing the carbon black of an average particle size less than 30 μm and the zircon of 40 μm with C/ZrSiO₄ mass ratios of 0.2, 0.3, 0.4, and 0.5. The prepared samples were subjected to the CTRN process at temperatures of 1673, 1723, 1753, and 1773 K for 6, 9, and 12 h. The CTRN process was conducted in an atmosphere-controlled tubular furnace in a nitrogen gas flow of 1.0 L/min. All the products were examined by X-ray powder diffraction to determine the transformation. The results showed that the proper transformation of ZrN-Si₃N₄ occurred at 1773 K for 12 h with a C/ZrSiO₄ mass ratio of 0.4.     

Growth Kinetics and Microstructure Evolution of Intermediate Phases in MoSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-WSi<Subscript>2</Subscript>/Mo Diffusion Couples

The growth kinetics and silicon diffusion coefficients of intermediate silicide phases in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple prepared by spark plasma sintering were investigated in temperatures ranging from 1200 to 1500 °C. The intermediate silicide phases were characterized by x-ray diffraction. The microstructures and components of the MoSi₂-Si₃N₄-WSi₂/Mo composites were investigated using scanning electron microscope with energy-dispersive spectroscopy. A special microstructure with MoSi₂ core surrounded by a thin layer of (Mo,W)Si₂ ring was found in the MoSi₂-Si₃N₄-WSi₂ composites. The intermediate layers of Mo₅Si₃ and (Mo,W)₅Si₃ in the MoSi₂-Si₃N₄-WSi₂/Mo diffusion couples were formed at different diffusion stages, which grew parabolically. Activation energy of the growth of intermediate layers in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple was calculated to be 316 ± 23 kJ/mol. Besides, the hindering effect of WSi₂ addition on the growth of intermediate layers was illustrated by comparing the silicon diffusion coefficients in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo and MoSi₂-3.5 vol.% Si₃N₄/Mo diffusion couples. MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂ coating on Mo substrate exhibited a better high-temperature oxidation resistance in air than that of MoSi₂-3.5 vol.% Si₃N₄ coating.     

Fabrication by Electrophoretic Deposition of Nano-Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@SiO<Subscript>2</Subscript> 3D Structure on Carbon Fibers as Supercapacitor Materials

Core–shell nanostructured magnetic Fe₃O₄@SiO₂ with particle size ranging from 3 nm to 40 nm has been synthesized via a facile precipitation method. Tetraethyl orthosilicate was employed as surfactant to prepare core–shell structures from Fe₃O₄ nanoparticles synthesized from pomegranate peel extract using a green method. X-ray diffraction analysis, Fourier-transform infrared and ultraviolet–visible (UV–Vis) spectroscopies, transmission electron microscopy, and scanning electron microscopy with energy-dispersive spectroscopy were employed to characterize the samples. The prepared Fe₃O₄ nanoparticles were approximately 12 nm in size, and the thickness of the SiO₂ shell was?~?4 nm. Evaluation of the magnetic properties indicated lower saturation magnetization for Fe₃O₄@SiO₂ powder (~?11.26 emu/g) compared with Fe₃O₄ powder (~?13.30 emu/g), supporting successful wrapping of the Fe₃O₄ nanoparticles by SiO₂. As-prepared powders were deposited on carbon fibers (CFs) using electrophoretic deposition and their electrochemical behavior investigated. The rectangular-shaped cyclic voltagrams of Fe₃O₄@CF and Fe₃O₄@C@CF samples indicated electrochemical double-layer capacitor (EDLC) behavior. The higher specific capacitance of 477 F/g for Fe₃O₄@C@CF (at scan rate of 0.05 V/s in the potential range of ??1.13 to 0.45 V) compared with 205 F/g for Fe₃O₄@CF (at the same scan rate in the potential range of?~???1.04 to 0.24 V) makes the former a superior candidate for use in energy storage applications.     

Properties and rapid consolidation of nanocrystalline 8MoSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> by high frequency induction heated sintering

Nanopowders of MoSi₂ and Si₃N₄ were synthesized from Mo₂N and Si by high ball milling. Dense nanostructured 8MoSi₂-Si₃N₄ composite was consolidated by high frequency induction heated sintering (HFIHS) method within two minutes from mechanically synthesized powders of MoSi₂ and Si₃N₄. Highly dense 8MoSi₂-Si₃N₄ composite with a relative density of up to 97 % was produced under simultaneous application of 80 MPa pressure and the induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.     

Compressive Strength Evaluation in Brazed ZrO<Subscript>2</Subscript>/Ti6Al4V Joints Using Finite Element Analysis

This study aims to synthesize and evaluate the compressive strength of the ZrO₂/Ti-6Al-4V joint brazed using an active metal filler Ag-Cu-Sn-Ti, and its application to dental implants assuring its reliability to resist the compressive failure in the actual oral environment. The brazing was performed at a temperature of 750 °C for 30 min in a vacuum furnace under 5 × 10^?6 Torr atmosphere. The microstructure of the brazed joint showed the presence of an Ag-rich matrix and a Cu-rich phase, and Cu-Ti intermetallic compounds were observed along the Ti-6Al-4V bonded interface. The compressive strength of the brazed ZrO₂/Ti-6Al-4V joint was measured by EN ISO 14801 standard test method. The measured compressive strength of the joint was ~1477 MPa—a value almost five times that of existing dental cements. Finite element analysis also confirmed the high von Mises stress values. The compressive strains in the samples were found concentrated near the Ti-6Al-4V position, matching with the position of the real fractured sample. These results suggest extremely significant compressive strength in ZrO₂/Ti-6Al-4V joints using the Ag-Cu-Sn-Ti filler. It is believed that a highly reliable dental implant can be processed and designed using the results of this study.     

Formation of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Diffusion Barrier Between Cr-Contained Stainless Steel and Cold-Sprayed Ni Coatings at High Temperature

A novel approach to prepare a coating system containing an in situ grown Cr₂O₃ diffusion barrier between a nickel top layer and 310SS was reported. Cold spraying was employed to deposit Ni(O) interlayer and top nickel coating on the Cr-contained stainless steel substrate. Ni(O) feedstock was prepared by mechanical alloying of pure nickel powders in ambient atmosphere, acting as an oxygen provider. The post-spray annealing was adopted to grow in situ Cr₂O₃ layer between the substrate and nickel coating. The results revealed that the diffusible oxygen can be introduced into nickel powders by mechanical alloying. The oxygen content increases to 3.25 wt.% with the increase of the ball milling duration to 8 h, while Ni(O) powders maintain a single phase of Ni. By annealing the sample in Ar atmosphere at 900 °C, a continuous Cr₂O₃ layer of 1-2 μm thick at the interface between 310SS and cold-sprayed Ni coating is formed. The diffusion barrier effect evaluation by thermal exposure at 750 °C shows that the Cr₂O₃ oxide layer effectively suppresses the outward diffusion of Fe and Cr in the substrate effectively.